1. Field of the Invention
This invention relates to transmissions and, more particularly, to transmissions which positively engage in one direction and are freewheeling in the opposite direction.
2. Description of the Related Art
Conventional bicycle transmissions allow power to be transmitted to the rear wheel while pedaling and freewheeling when pedaling stops. Typically, such transmissions, called freewheeling transmissions, use a ratchet wheel and a plurality of pawl components aligned radially. The pawls are pivotally mounted on a coaxially-mounted member which act against the teeth on a ratchet wheel. When pedaling, the outer tip of the pawls engage the teeth on the ratchet wheel to force the ratchet wheel in one direction. When pedaling is stopped or reversed, the tips of the pawls disengage the teeth of the ratchet wheel thereby enabling the transmission to freewheel. When freewheeling, the tips of the pawls move over the teeth on the ratchet wheel as it continues to rotate, creating the familiar "clicking" sound of a coasting bicycle. Although the "clicking" sound can be annoying to some users, ratchet and pawl type transmissions are compact, cheap, lightweight and can transmit considerable torque for their size and weight.
With the growing popularity of mountain riding and on and off-road racing, the demand for higher performance transmissions with greater torque capacity has increased. Unfortuately, traditional ratchet and pawl type transmissions are unable to meet this demand. Although the easiest and best means to increase the torque capacity of the ratchet and pawl type transmissions would be to increase the diameter of the rear hub or the number of pawls, world-wide adopted standards governing rear hub sizes and the exchangability of rear wheel sprocket sets preclude such change. Also, by increasing the number of pawls, friction is increased in the freewheel direction.
Another important drawback with ratchet and pawl type transmissions is that they require proper lubrication and cleaning. This is especially true for downhill racing and off-road riding. To overcome this problem, most ratchet and pawl type transmissions use labyrinth or very lightly loaded lip seals to prevent contamination which have proven to be only marginally effective.
Another drawback with ratchet and pawl type transmissions is the noise created while coasting. Such noise can be a tactical liability when racing.
Recently, a modified ratchet and pawl type transmission has been developed by Union Sils of Germany which addresses some of the problems associated with typical ratchet and pawl type transmissions. In the modified ratchet and pawl type transmission, two axially aligned, toothed discs face each other, one fixed to a driver and a other splined to the driven member. A spring behind the axially free disc lightly loads it against the driven disc. In the torque-carrying direction, the discs are forced together which enables their opposing sets of teeth to engage and transmit torque. In reverse rotation, the shape of the teeth forces the moveable disc to separate from the fixed disc to disengage therefrom. As rotation continues, the spring forces the moveable disc towards the fixed disc thereby producing an audible "clicking" sound. Unfortunately, substantial drag is produced when in the freewheel direction. Also, failure is possible due to the high speed oscillary motion of the driven disc while freewheeling. This disc has many times the mass of the traditional pawls and, in combination with a weak return spring, has a low resonant frequency. This could damage parts or cause power robbing resonant modes of behavior within the speed range of a bicycle.
Another type of transmission used for bicycles is known as a uni-directional friction clutch as disclosed by the manufacturers, Winner and Torrington. Such transmissions exhibit low drag and quiet operation in the freewheel direction, and engage quickly in the torque-carrying direction. Their principle weakness, however, is a lack of torque carrying ability, without incurring unacceptable penalties in size and weight.